A liquid crystal display device has been widely utilized for a display of a personal computer, a television or the like. The device utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal compound. As an operating mode of the liquid crystal display device, such a mode is known as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (WS) mode and a polymer sustained alignment (PSA) mode.
In such a liquid crystal display device, a liquid crystal composition having suitable physical properties is used. In order to further improve characteristics of the liquid crystal display device, the liquid crystal compound contained in the composition preferably has physical properties described in (1) to (9) below.
(1) High stability to heat, light and so forth,
(2) a high clearing point,
(3) low minimum temperature of a liquid crystal phase,
(4) small viscosity (η),
(5) suitable optical anisotropy (Δn),
(6) large dielectric anisotropy (Δ∈),
(7) a suitable elastic constant (K),
(8) excellent compatibility with other liquid crystal compounds, and
(9) a large dielectric constant in a minor axis direction (∈⊥).
An effect of the physical properties of the liquid crystal compound on the characteristics of the device is as described below. A compound having the high stability to heat, light and so forth as described in (1) increases a voltage holding ratio of the device. Therefore, a service life of the device becomes long. A compound having the high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having the low minimum temperature of the liquid crystal phase such as the nematic phase and a smectic phase, as described in (3), in particular, a compound having the low minimum temperature of the nematic phase, also extends the temperature range in which the device can be used. A compound having the small viscosity as described in (4) shortens a response time of the device.
A compound having the suitable optical anisotropy as described in (5) improves contrast of the device. According to a design of the device, a compound having a large optical anisotropy or a small optical anisotropy, more specifically, a compound having the suitable optical anisotropy, is required. When the response time is shortened by decreasing a cell gap of the device, a compound having the large optical anisotropy is suitable. A compound having the large dielectric anisotropy as described in (6) decreases a threshold voltage of the device. Thus, an electric power consumption of the device is decreased. On the other hand, a compound having a small dielectric anisotropy shortens the response time of the device by decreasing viscosity of the composition.
With regard to (7), a compound having a large elastic constant decreases the response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Therefore, a suitable elastic constant is required according to characteristics to be desirably improved. A compound having the excellent compatibility with other liquid crystal compounds as described in (8) is preferred. The reason is that the physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
Further, an improvement of a transmittance in the liquid crystal composition has been strongly required in connection with a demand for achieving a low power consumption and a high definition in the liquid crystal display device in recent years. Above all, the transmittance in the liquid crystal composition used for an FFS mode liquid crystal display device is known to be correlated with the dielectric constant in the minor axis direction (∈⊥) of the liquid crystal composition, and therefore a liquid crystal compound having the large dielectric constant in the minor axis direction as described in (9) is preferred.
A variety of liquid crystal compounds each having a CF2O bonding group have so far been prepared as the liquid crystal compound having the large dielectric anisotropy, and some of the compounds have been practically used. However, in the above compounds, the dielectric constant in the minor axis direction is far from sufficiently large. Under such circumstances, desire has been expressed for development of a compound having excellent physical properties and a suitable balance regarding the physical properties (1) to (9) above, above all, a compound having both the large dielectric anisotropy (Δ∈) and the large dielectric constant in the minor axis direction.